The present invention relates generally to multilayer barriers, and more particularly to multilayer barriers stacks having improved properties.
Many devices are subject to degradation caused by permeation of environmental gases or liquids, such as oxygen and water vapor in the atmosphere or chemicals used in the processing of the electronic product. The devices are usually encapsulated in order to prevent degradation.
Various types of encapsulated devices are known. For example, U.S. Pat. No. 6,268,695, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Jul. 31, 2001; U.S. Pat. No. 6,522,067, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making,” issued Feb. 18, 2003; and U.S. Pat. No. 6,570,325, entitled “Environmental Barrier Material For Organic Light Emitting Device And Method Of Making”, issued May 27, 2003, all of which are incorporated herein by reference, describe encapsulated organic light emitting devices (OLEDS). U.S. Pat. No. 6,573,652, entitled “Encapsulated Display Devices”, issued Jun. 3, 2003, which is incorporated herein by reference, describes encapsulated liquid crystal displays (LCDs), light emitting diodes (LEDs), light emitting polymers (LEPs), electronic signage using electrophoretic inks, electroluminescent devices (EDs), and phosphorescent devices. U.S. Pat. No. 6,548,912, entitled “Semiconductor Passivation Using Barrier Coatings,” issued Apr. 15, 2003, which is incorporated herein by reference, describes encapsulated microelectronic devices, including integrated circuits, charge coupled devices, light emitting diodes, light emitting polymers, organic light emitting devices, metal sensor pads, micro-disk lasers, electrochromic devices, photochromic devices, microelectromechanical systems, and solar cells.
Generally, encapsulated devices can be made by depositing barrier stacks adjacent to one or both sides of the device. The barrier stacks typically include at least one barrier layer and at least one decoupling layer. There could be one decoupling layer and one barrier layer, there could be multiple decoupling layers on one side of one or more barrier layers, or there could be one or more decoupling layers on both sides of one or more barrier layers. The important feature is that the barrier stack has at least one decoupling layer and at least one barrier layer.
One embodiment of an encapsulated display device is shown in FIG. 1. The encapsulated display device 100 includes a substrate 105, a display device 110, and a barrier stack 115. The barrier stack 115 includes a barrier layer 120 and a decoupling layer 125. The barrier stack 115 encapsulates the display device 110, preventing environmental oxygen and water vapor from degrading the display device.
The barrier layers and decoupling layers in the barrier stack can be made of the same material or of a different material. The barrier layers are typically about 100-400 Å thick, and the decoupling layers are typically about 1000-10,000 Å thick.
Although only one barrier stack is shown in FIG. 1, the number of barrier stacks is not limited. The number of barrier stacks needed depends on the level of water vapor and oxygen permeation resistance needed for the particular application. One or two barrier stacks should provide sufficient barrier properties for some applications, while three or four barrier stacks are sufficient for most applications. The most stringent applications may require five or more barrier stacks.
The decoupling layers can be deposited using a vacuum process, such as flash evaporation with in situ polymerization under vacuum, or plasma deposition and polymerization, or atmospheric processes, such as spin coating, ink jet printing, screen printing, or spraying. Suitable materials for the decoupling layer include, but are not limited to, organic polymers, inorganic polymers, organometallic polymers, hybrid organic/inorganic polymer systems, and silicates.
The barrier layers can be deposited using a vacuum process, such as sputtering, chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD), plasma enhanced chemical vapor deposition (PECVD), evaporation, sublimation, electron cyclotron resonance-plasma enhanced vapor deposition (ECR-PECVD), and combinations thereof. Suitable barrier materials include, but are not limited to, metals, metal oxides, metal nitrides, metal carbides, metal oxynitrides, metal oxyborides, and combinations thereof.
In general, the deposition of dense layers with suitable barrier properties is achieved by supplying energy to the species impinging on the substrate to form the layer. Such energy may be supplied as thermal energy. In some of the depositions mentioned above, more specifically the energy is supplied by using ionization radiation to increase the ion production in the plasma and/or to increase the number of ions in the evaporated material streams. The ionization radiation may be (UV) light, ion beam, electron beam, and electromagnetic field. The ions so produced are then accelerated to the substrate either by applying a DC or AC bias to the substrate, or by building up a potential difference between the plasma and the substrate.
We have found that some devices encapsulated with multilayer barrier stacks have been damaged by the plasma used in depositing the barrier and/or decoupling layers. Device plasma damage has occurred when a substrate with a plasma sensitive device on it, such as an OLED, is encapsulated with a multi-layer barrier stack in which a plasma based and/or assisted process is used to deposit a barrier layer and/or decoupling layer on the OLED. For example, device plasma damage has occurred when reactively sputtering a barrier layer of AlOx under conditions suitable for achieving barrier properties, sputtering a barrier layer of AlOx onto the top surface of a plasma sensitive device, and/or sputtering a barrier layer of AlOx on a vacuum deposited, acrylate based polymeric layer.
Device plasma damage associated with the use of plasma in the deposition of a barrier layer, a decoupling layer, or another layer on a device essentially has a negative impact on the electrical and/or luminescent characteristics of the encapsulated device. The effects will vary by the type of device, the structure of the device, and the wavelength of the light emitted by the OLED. It is important to note that device plasma damage is dependent on the design of the device to be encapsulated. For example, Olds made by some manufacturers show little to no device plasma damage, while Olds made by other manufacturers show significant device plasma damage under the same deposition conditions. This suggests that there are features within the device that affect its sensitivity to plasma exposure.
One way to detect this type of device plasma damage is to measure the voltage needed to achieve a specified level of luminescence. Another way is to measure the intensity of the luminescence. Device plasma damage results in higher voltage requirements to achieve the same level of luminescence (typically 0.2 to 0.5 V higher for an OLED), and/or lower luminescence.
Although not wishing to be bound by theory, device plasma damage that is observed when a decoupling layer employing plasma, a sputtered AlOx, or another layer employing plasma is formed (deposited) directly on an OLED or other sensitive device is believed to be due to an adverse interaction between the device and one or more components of the plasma, including ions, electrons, neutral species, UV radiation, and high thermal input.
This type of device plasma damage and methods of reducing it are described in application Ser. No. 11/439,474, filed May 23, 2006, entitled Method of Making an Encapsulated Plasma Sensitive Device.
In addition, it is known that plasma treatments can modify the properties of polymers. Several patents disclose the use of plasma treatment to improve properties for a multilayer barrier on a substrate. U.S. Pat. No. 6,083,628 discloses plasma treatment of polymeric film substrates and polymeric layers from acrylates deposited using a flash evaporation process as a way to improve properties. U.S. Pat. No. 5,440,466 also discusses plasma treatment of substrates and acrylate layers to improve adhesion. The improvement in adhesion is the result of breaking chemical bonds and creating new chemical species on the surface of the substrate. On the other hand, it is known that, in some cases, plasma and/or radiation exposure degrades the functional properties of polymers (polymer plasma damage).
It would be desirable to eliminate processes which use plasma in manufacturing barrier stacks and/or devices including barrier stacks. However, avoiding such processes is not always possible.
Therefore, there is a need for improved deposition conditions that reduce or eliminate damage to the polymeric decoupling layer, and for multilayer barriers having polymeric decoupling layers with reduced damage.